On Monday, Libya demanded an explanation from the US ambassador over the arrest of Mr Liby, who is wanted in the US over the 1998 bombings of US embassies in Kenya and Tanzania.

A number of militia groups operate in Libya - they are nominally attached to government ministries but often act independently and, correspondents say, often have the upper hand over police and army forces.

The government has been struggling to contain these militia, who control parts of the country, two years after the revolt which overthrew Muammar Gaddafi.

Cabinet summoned

Mr Zeidan was taken by armed men from Tripoli's Corinthia Hotel - considered one of the more secure buildings in the city - in the early hours of Thursday morning.

The head of security at the hotel, Abd al-Razaq ben Shaban, told Reuters news agency "revolutionaries" had arrived carrying papers with an order from the prosecutor general for the arrest of the prime minister.

However, state-run National Libyan TV quoted Justice Minister Salah al-Marghani as saying that the prosecutor general had issued no warrant for Mr Zeidan's arrest.

The Revolutionaries Operations Room said it had seized Mr Zeidan, and was acting on the orders of the prosecutor general in accordance with Libya's criminal code.

Militias in Libya

Numerous militias formed to topple Gaddafi still operate

Many still control the towns or areas where they were formed

Some believed to have links to al-Qaeda

Government has been unable to disarm them, instead it works with some militias

Libya Revolutionaries Operations Room, which says it seized the prime minister, has links to defence and interior ministries

A spokesman for the group told Reuters that they took Mr Zeidan following comments by US Secretary of State John Kerry that the Libyan government had been "aware of the operation".

The Libyan government said on its website earlier that Mr Zeidan had been taken from a hotel "to an unknown place for unknown reasons by a group thought to be from the Tripoli Revolutionaries Operations Room and the Committee for Fighting Crime".

Al-Arabiya TV station broadcast images on Thursday morning which showed Mr Zeidan looking dishevelled and being escorted by what the station said were armed men.

Libya's cabinet has been summoned for an immediate meeting under the leadership of the deputy prime minister.

UK Foreign Secretary William Hague condemned the capture and called for Mr Zeidan's immediate release.

"It is vital that the process of political transition in Libya is maintained. The government and people of Libya have our full support at this concerning time," he said.

US state department spokeswoman Jen Psaki said the US was looking into the reports and was "in close touch with senior US and Libyan officials on the ground".

Anas al-Liby

Born 30 March 1964 in Tripoli, Libya. Also known as Nazih Abdul-Hamed al-Ruqai

Believed to have joined al-Qaeda in 1990s

Given political asylum in UK

Rumoured to have returned to Libya during 2011 civil war

Charged by New York prosecutors in 2000 with involvement in the 1998 Kenya and Tanzania US embassy bombings